Legislation mandating stringent limits on certain exhaust emissions from motor vehicle internal combustion engines has resulted in widespread use of closed-loop fuel control systems and three-way catalytic converters for oxidizing hydrocarbons and carbon monoxide while simultaneously reducing oxides of nitrogen in the exhaust gas. Exhaust emissions are influenced by, among other things, the air/fuel ratio of the engine fuel charge, combustion temperature and converter temperature.
The air/fuel ratio of the engine fuel charge affects the efficiency of three-way catalytic converters and, therefore, influences the exhaust emissions. Thus, a vehicle so equipped is also equipped with a closed-loop fuel control system for maintaining a stoichiometric air/fuel ratio which maximizes the efficiency of catalytic converter operation. The exhaust emissions of such vehicles are also influenced by combustion temperature and converter temperature. Elevated combustion temperature is known to increase the production of oxides of nitrogen in the exhaust gases. However, elevated converter temperature is desirable for proper converter operation; but converter temperature above a certain limit may damage the converter and thus permanently reduce its efficiency. Exhaust gas temperature is a good indicator of both combustion temperature and converter temperature, since it is a direct result of the former and a significant contributor to the latter. An exhaust gas temperature sensor may, therefore, be useful for additional control or monitoring functions in engine controls using three-way catalytic converters and closed-loop fuel control systems. In prior art devices, dedicated thermocouples or thermistors have been employed for sensing exhaust gas temperature; but their addition, along with required external amplification circuitry, can significantly increase the expense of a fuel control system intended for mass production.